1. Field of the Invention
The present invention relates to a dielectric laminated filter, and more particularly, to a dielectric laminated filer able to improve a skirt characteristic of a resonance frequency by controlling a location of a resonating point generating according to an electronic combination between resonators.
2. Description of the Related Art
According to a recent development of a radio wave technology, demands of wireless communication equipment or mobile telecommunication terminal increase. A characteristic of these wireless apparatus depends on a filtering characteristic of a filter used in the wireless apparatus.
The filter used for filtering a radio wave is classified into one of a saw filter and a dielectric filter. Although the saw filter is small in volume, a cost is high, and it is very difficult to be realized in a high frequency beyond an S band whereas the dielectric filter is too bulky in volume although the cost is low.
The dielectric filter is classified into one of a bulk type dielectric filter and a laminated type dielectric filter. The bulky type dielectric filter, which has been widely used, cannot be used in a minimized telecommunication apparatus. In the laminated type dielectric filter, an attenuation characteristic is lowered in a frequency near a transmissive band compared to the saw filter and the bulk type dielectric filter. However, the laminated type dielectric filter has been developed to have an excellent filtering function, be minimized, and become lightweight since the laminated type dielectric filter has an excellent spurious characteristic and is small in volume.
FIG. 1 is a perspective view of a conventional dielectric laminated filter 1. Referring to FIG. 1, the dielectric laminated filter 1 includes a dielectric block 2 having a hexahedron and laminated with a plurality of dielectric sheets, input and output electrodes 3, 4 formed on first opposite sides of the dielectric body 2, and ground electrodes 5a, 5b formed on second opposite sides of the dielectric body 2.
The dielectric block 2 is made of the dielectric sheets which are laminated, various patterns are formed on respective dielectric sheets. FIGS. 2A and 2B are cross-sectional views taken along lines A—A and B—B of FIG. 1, respectively, to show pattern arrangements of the dielectric sheets of the dielectric block.
As shown in FIGS. 2A and 2B, the dielectric laminated filter 1 includes ground patterns 6a, 6b coupled to the ground electrodes 5a, 5b, resonator patterns 9 disposed between the ground patterns 6a, 6b, having one end coupled to the ground electrode 5a, and disposed parallel to one another in a plane, and input and output patterns 10 to which two of the resonator patterns 9 disposed on both sides of the plane are coupled, respectively. A plurality of load capacitor patterns 7, 8 are arranged to be parallel to the resonator patterns 9 between the resonator patterns 9 and the ground patterns 6a, 6b. The load capacitor patterns 7, 8 are coupled to the ground pattern 5b at their end. The respective patterns are formed to be spaced-apart from each other by a predetermined distance, and a dielectric material is filled in spaces between the respective patterns.
FIG. 3 is an exploded view of pattern structures of the dielectric laminated filter shown in FIG. 1, and FIG. 4 is an equivalent circuit diagram of the dielectric laminated filter shown in FIG. 1. Referring to FIGS. 3 and 4, the resonator patterns 9 (9a, 9b, 9c) form resonators R1, R2, R3 coupled to a ground at their one end, and the load capacitor patterns 7, 8 disposed above and below the resonator patterns 9 and parallel to the resonator patterns 9 form load capacitors CR1, CR2, CR3, coupled to resonators R1, R2, R3 to be parallel to the resonators R1, R2, R3. Respective electronic couplings between input and output electrodes 3, 4 and the resonator patterns 9 and between the resonator patterns 9 form a plurality of inductance couplings L01, L02, L03, L04 to show an equivalent characteristic of the equivalent circuit shown in FIG. 4.
In the dielectric laminated filter 1 having the above structure, a location of a resonating point of the dielectric body 2 is determined according to the load capacitors CR1, CR2, CR3 and the resonators R1, R2, R3, the dielectric body has a transmissive characteristic on signals of a predetermined frequency band based on the resonating point.
However, a response characteristic of the above structure of the dielectric laminated filter 1 shows that a skirt characteristic of a high frequency portion (a right side) of the predetermined frequency band deteriorates.
In order to improve the skirt characteristic of the dielectric laminated filter or adjust the skirt characteristic according to a user demand, the number of the resonators is increased according to an increase of the number of filter sections, or a method of forming an attenuation pole near the transmitting frequency band.
If the number of the resonators is increased, an insertion loss occurs due to an increased number of the resonators, and it is limited to increase the number of the filter sections within a limited size of the dielectric body.
Although the method of forming the attenuation pole near the transmitting frequency band may increase the skirt characteristic without the increase of the number of the filter sections, an additional circuit is required to form the attenuation pole, thereby causing a filter circuit to be complicated.
In addition, if the dielectric laminated filter is minimized, a coupling generated between circuit patterns inserted into the dielectric body to form the attenuation pole is generated to distort a filter characteristic of the dielectric laminated filter.